As the development of the laser processing technology, the laser machine for high-power cutting is extensively applied to the field of heavy industry. In order to cut a thick (about 50 mm) steel plate, a high-power laser is required. At present, the high-power laser which is mature and has a highest performance cost ratio is the CO2 laser. The laser has a power reaching 5000-10000 watts, or even greater. However, the working wavelength of the CO2 laser is too long, such as the wavelength λ=10640 nm (10.64 μm); it can be concluded according to the Rayleigh rule that:
The theoretic resolution distance for laser cutting d=2.44λf/D.
Wherein d represents a minimum resolution distance between two points;
λ represents a wavelength of the processing laser;
f represents a focal length of the optical lens;
D represents an entrance pupil diameter of the optical lens.
The wavelength is 10 times of a common use wavelength 1064 nm, the resolution ratio of the laser is just one-tenth of the wavelength 1064 nm, causing the cutting accuracy to reduce. As shown in above formula, if the resolution ratio is to be enhanced, the focal length is required to decrease or the entrance pupil diameter is required to increase. However, increasing the entrance pupil diameter leads to a sharply increasing of the volume of the lens sharply, and enhancement of the resolution is not so obvious. Therefore, under the premise of controlling the volume, meeting a requirement of high resolution remains a design problem of the Fθ lens for far-infrared laser cutting.